The present invention relates to optical signal transcription devices using an information support medium illuminated by a light spot for writing or reading digital data or similar along a pre-engraved track.
The disk-shaped support medium on which this information is to be written has been pre-engraved, for example in the form of a groove, with a set of track elements arranged in concentric circles or organized along the turns of a spiral. The width of the track elements is chosen slightly less than the diameter of the light spot and these elements are separated by inter-track areas whose width is slightly greater than this diameter. The relief surface of the support medium receives a thin layer appropriate to writing by thermo-optical means. This arrangement of elements allows the track to be scanned before writing, for interaction of the reading spot with the track causes a dispersed radiation of reduced intensity, whereas the inter-track areas do not generate any dispersion.
It is, from the detection point of view, as if the tracks were darker than the inter-tracks. Any deviation from following the track may then be easily detected because of this amplitude contrast. When the layer covering the support medium is subjected to the writing radiation, the written zones are illuminated, which contributes to creating a good contrast along the track. However, these illuminated zones tend to merge with the lower density of the adjacent intertracks, which causes the contrast to be lost radially which is required for good track following.
It can then be seen that writing adversely affects good radial location of the tracks in all the zones where the layer has stored data. On the other hand, the track-intertrack contrast remains good in the zones free from writing.
The above-mentioned device requires then that a track carried by the information support means be followed, the invention relates more particularly to the means for following said track. These means are used to ensure scanning of the track by means of the light spot and to this end they comprise optical detection of the deviations from track following and a reading spot displacement member actuated so as to compensate for the deviations.
An optical transcription devices usually comprises a radiation source, an optical modulating means with electrical control and means for allowing the track to be read or recorded to be followed. The radiation source provides a beam of sufficient strength for recording on the information support medium. The beam at the output of the optical modulating means is modulated in accordance with the signals to be written present at the input of the modulating means. The track following means comprise at least one detection means in which the radiation gives rise to a signal which acts on the beam deflector means for ensuring following of the track.
Scanning of the pre-engraved blank track raises no difficulties but it is different during writing and during reading taking place after the data have been written.
In fact, when data are recorded, the intensity of illumination of the support medium changes value so that the radiation reflected or transmitted by the information support medium undergoes intensity modulation which does not exist in the reading mode. Writing takes place generally with beam intensities considerably higher than those required for reading, which causes a sudden variation in the gain of the feedback loop which controls following of the track. In addition, during these sudden changes in intensity level, transitory phenomena are created which lead to instabilities prejudicial to correct operation of the feed-back loop.
When recorded tracks are read, a series of illuminated zones are detected intersected by darker zones, but radially the track-intertrack contrast becomes uneven since only the unrecorded zones of the track have kept their initial contrast with respect to the intertracks.
The present invention aims at obviating these drawbacks by adopting a sampling technique making track following insensitive to the modulation of a writing beam.